Thermal printers found in the prior art have considerable difficulty in reliably producing a large number of grey scale values. In my co-pending application Ser. No. 607,986, filed 5-7-84 entitled "Shaped Head For Thermal Recording", and assigned to the assignee of the present invention, I disclose and claim a heater element in the form of an "hour glass" shaped resistor capable of producing a large number of different area melts, for example, 32 different grey scale values on a thermal recording medium such as a dye transfer paper and receiving sheet.
An assembly of these shaped heaters might have, for example, 2,728 of these resistors mounted in a row on a 9 to 10 inch bar so as to be useable vertically or horizontally with an 81/2.times.11 inch of paper having white borders surrounding the image area. Such an assembly would produce 2,048 lines in the image area and if every pixel can be dependend upon for 32 precise melt sizes from each shaped heater, a palet of over 32,000 colors would be provided by each pixel, assuming the pixels are produced during three passes of the head over the paper with each pass forming one of the colors cyan, magenta, and yellow.
To obtain 32 melt sizes per pixel the area change of the melt is 3.23% for each increment going from 0 to 31 as the full pixel melt. Any single disturbance such as variations of temperature at the start of programmed heating should cause no more error than about 1% of the difference in size between the smallest dot and the largest dot, i.e., less than half of the 3.23% density change between two adjacent grey scale values. This precision is needed over the range of normal office temperatures. It is also preferable that there be a minimum warmup time required for the system.
Present two grey scale systems show objectional changes in melt size depending on past heater commands, operating time, office ambient temperature, etc. Ishibashi in U.S. Pat. No. 4,284,876 varies the width of a resistor heating pulse so as to increase or decrease the heating thereof based on the past history of heating pulses. Ishibashi is compensating for only 32 resistors, requires only one memory bit for each past history time per resistor and is not attempting a 1% compensation. Applying this approach for 2,728 resistors with 5 bits required for each history point per resistor with the other associated hardware the cost and complexity would be excessive. Furthermore, Ishibashi does not address the ambient temperature and warmup problems.
Cunningham et al. in U.S. Pat. No. 4,305,080 uses an R/C network in each heater circuit to approximate past heating history of a resistor and set the next voltage pulse width accordingly. This might work with a two-level grey scale system, but it will not work with an "hour glass" shaped heater producing 32 melt sizes in one pulse and the 32 size system has several system nonlinearities that Cunningham cannot take into account.
For a black and white system, Anno et al., in U.S. Pat. No. 4,364,063, make corrections in pulse width based on the absence or presence of a previous pulse. However, his system cannot handle the nonlinearities of the 32 grey scale system. Ambient temperature correction is not provided in the Cunningham and Anno systems.
Minowa, in U.S. Pat. No. 4,113,391, describes a battery operated system with 7 resistors to produce two grey scales. Trouble with usable optical density occurs when the batteries run down, so the pulse width of all pulses are varied along with motor speed depending on battery voltage and temperature. There is no pulse width control based on thermal history.
Brescia, et al. in U.S. Pat. No. 3,777,116, describes a 7 heater two grey scale system. To avoid overheating of the resistors, the overall voltage supply for the resistors is controlled by the duty cycle of the resistors and individual resistor voltages are controlled by individual use rate. Although the duty cycle of all resistors may be thought of as a proxy for head temperature, there is no measurement of head temperature. This approach is certainly not applicable to a system which needs 1% temperature accuracy.
Ito et al., in U.S. Pat. No. 3,975,707, describes a 4.times.5 heater matrix where ambient temperature controls the on time of addressed heaters. No past history correction is provided.